Method of surface oxidizing zirconium alloys and resulting product

ABSTRACT

An oxide coating is formed on zirconium or zirconium alloys of refined microstructure by a process comprising at least the step of inducing an appropriate altered surface roughness such that subsequent oxidation results in a uniformly thick oxide coating. An oxide coating of uniform and controlled thickness is especially useful on orthopedic implants of zirconium or zirconium-based alloys to provide low friction, highly wear resistant surfaces on artificial joints, such as hip joints, knee joints, elbows, etc. The uniformly thick oxide coating of controlled depth on oxide coated prostheses provides a barrier against implant corrosion caused by ionization of the metal prosthesis.

This application claims priority to international applicationPCT/US98/06059, filed Mar. 27, 1998 and to U.S. provisional applicationNo. 60/042,364, filed Mar. 27, 1997.

This invention relates to metallic orthopedic implants with load bearingsurfaces coated with a thin, dense, low friction, highly wear-resistant,uniformly thick coating of zirconium oxide.

The invention also relates to uniformly thick zirconium oxide coatingson the non-load bearing surfaces of an orthopedic implant where thezirconium oxide provides a barrier between the metallic prosthesis andbody tissue thereby preventing the release of metal ions and corrosionof the implant.

The invention also relates to a method of producing a uniformly thickoxide coating on zirconium or a zirconium alloy by controlling thesurface roughness of the zirconium or zirconium alloy having a refinedmicrostructure prior to formation of the oxide coating.

The excellent corrosion resistance of zirconium has been known for manyyears. Zirconium displays excellent corrosion resistance in many aqueousand non-aqueous media and for this reason has seen an increased use inthe chemical process industry and in medical applications. A limitationto the wider application of zirconium in these areas is its relativelylow resistance to abrasion and its tendency to gall. This relatively lowresistance to abrasion and the tendency to gall is also demonstrated inzirconium alloys.

Orthopedic implant materials must combine high strength, corrosionresistance and tissue compatibility. The longevity of the implant is ofprime importance especially if the recipient of the implant isrelatively young because it is desirable that the implant function forthe complete lifetime of a patient. Because certain metal alloys havethe required mechanical strength and biocompatibility, they are idealcandidates for the fabrication of prostheses. These alloys include 316Lstainless steel, chrome-cobalt-molybdenum alloys and, more recently,titanium alloys which have proven to be the most suitable materials forthe fabrication of load-bearing prostheses.

One of the variables affecting the longevity of load-bearing implants,such as hip joint implants, is the rate of wear of the articulatingsurfaces and long-term effects of the metal ion release. A typical hipjoint prosthesis includes a stem, a femoral head and an acetabular cupagainst which the femoral head articulates. Wear of either or both ofthe articulating surfaces results in an increasing level of wearparticulates and “play” between the femoral head and the cup againstwhich it articulates. Wear debris can contribute to adverse tissuerejection leading to bone resorption, and ultimately the joint must bereplaced.

The rate of wear is dependent upon a number of factors which include therelative hardness and surface finish of the material which constitutethe femoral head and the acetabular cup, the frictional coefficientbetween the materials of the cup and head, the load applied and thestresses generated at the articulating surface. The most commom materialcombinations currently used in fabrication of hip joints implantsinclude femoral heads of cobalt or titanium alloys articulating againstacetabular cups lined with organic polymers or composites of suchpolymers including, e.g., ultra high molecular weight polyethylene(UHMWPE), and femoral heads of polished alumina in combination withacetabular cups lined with an organic polymer or composite or cups madeof polished alumina.

Of the factors that influence the rate of wear of conventional hip-jointimplants, the most significant are patient weight and activity level.Additionally, heat which is generated by friction in the normal use ofthe implant as, for instance, in walking has been shown to causeaccelerated creep and wear of the polyethylene cup. Furthermore, thereis a correlation between the frictional moment which transfers torqueloading to the cup and the frictional coefficient between the femoralhead and the surface of the acetabular cup against which the headarticulates. Cup torque has been associated with cup loosening. Thus, ingeneral, the higher the coefficient of friction for a given load, thehigher the level of torque generated. Ceramic bearing surfaces have beenshown to produce significantly lower levels of frictional torque.

It is also noteworthy that two of the three commonly used hip-jointsystems as indicated above include a metallic femoral head articulatingagainst a UHMWPE liner inside the acetabular cup. UHMWPE, being apolymeric material, is more susceptible to creep when heated than thecommonly used metal alloys or ceramics and is consequently moresusceptible to wear than the alloys or ceramics.

It has also been found that metal prostheses are not completely inert inthe body. Body fluids act upon the metals causing them to slowly corrodeby an ionizing process that thereby releases metal ions into the body.Metal ion release from the prosthesis is also related to the rate ofwear of load bearing surfaces because the passive oxide film, which isformed on the surface, is constantly removed. The repassivation processconstantly releases metal ions during the ionizing process. Furthermore,the presence of third-body wear (cement or bone debris) accelerates thisprocess and microfretted metal particles increase friction.Consequently, the UHMWPE liner inside the acetabular cup, against whichthe femoral head articulates, is subjected to accelerated levels ofcreep, wear and torque.

U.S. Pat. No. 415,764 to Suzuki, et al. recognizes that while metalprostheses have excellent mechanical strength they tend to corrode inthe body by ionization. Suzuki, et al. also recognized the affinitybetween ceramics and bone tissue but noted that ceramic prostheses areweak on impact resistance. Suzuki et al. therefore proposed metalprosthesis plasma sprayed with a bonding agent which is in turn coveredwith a porous cement coating which will allow the ingrowth of bonespincules into the pores. This combination, it was said, would provideboth the mechanical strength of metals and the bio-compatibility ofceranics.

The Suzuki patent did not address the issue of friction or wear oforthopedic implant bearing surfaces but confined itself to the singleissue of the biocompatibility of metal prostheses. Furthermore, Suzukiet al. did not address the issue of dimensional changes that occur whenapplying a coating or the effect of these dimensional changes in thetightness of fit between the surfaces of an articulating jointprosthesis.

In addition, the application of ceramic coating to metal substratesoften results in non-uniform, poorly adhering coatings which tend tocrack due to the differences in elastic modulus or thermal expansionbetween the ceramic and underlying metal substrate. Further,or; suchcoatings tend to be relatively thick (50-300 microns) and since the bondbetween the metal and the ceramic coating is often weak, there is therisk of galling or separation of ceramic coatings.

Previous attempts have been made to produce zirconium oxide coatings onzirconium pans for the purpose of increasing their abrasion resistance.One such process is disclosed in U.S. Pat. No. 3,615,885 to Watson whichdiscloses a procedure for developing thick (up to 0.23 mm) oxide layerson Zircaloy 2 and Zircaloy 4. However, this procedure results insignificant dimensional changes especially for parts having a thicknessbelow about 5 mm, and the oxide film produced does not exhibitespecially high abrasion resistance.

U.S. Pat. No. 2,987,352 to Watson discloses a method of producing ablue-black oxide coating on zirconium alloy parts for the purpose ofincreasing their abrasion resistance. Both U.S. Pat. No. 2,987,352 andU.S. Pat. No. 3,615,885 produce a zirconium dioxide coating on zirconiumalloy by means of air oxidation. U.S. Pat. No. 3,615,885 continues theair oxidation long enough to produce a beige coating of greaterthickness than the blue-black coating of U.S. Pat. No. 2,987,352. Thisbeige coating does not have not have the water resistance of theblue-black coating and is thus not applicable to many parts where thereare two work faces in close proximity. The beige coating wears down morequickly than the blue-black oxide coating with the resulting formationof zirconium oxide particles and the loss of the integrity of thezirconium oxide surface. With the loss of the oxide surface thezirconium metal is then exposed to its environment and can lead totransport of zirconium ions away from the surface of the metal into theadjacent environment.

The blue-black oxide coatings have a thickness which is less than thatof the beige coating although the hardness of the blue-black coating ishigher than that of the beige coating. This harder blue-black oxidecoating lends itself better to surfaces such as prosthetic devices.Although the blue-black coating is more abrasion resistant than thebeige coating it is a relatively thin coating. It is therefore desirableto produce the blue-black coatings of increased abrasion resistancewithout producing the same type coatings of the prior art.

U.S. Pat. No. 5,037,438 to Davidson discloses a method of producingzirconium alloy prostheses with a zirconium oxide surface. U.S. Pat. No.2,987,352 to Watson discloses a method of producing zirconium bearingswith a zirconium oxide surface. The oxide coating produced is not alwaysuniform in thickness and the non-uniformity reduces the integrity of thebonding between the zirconium alloy and the oxide layer and theintegrity of the bonding with the oxide layer.

There exists a need for a method to produce oxide coatings formthickness on zirconium alloys. There exists a need for a metal alloybased orthopedic implant having low friction and highly wear resistantload bearing surfaces that can be implanted for the lifetime of therecipient. There also exists a need for a metal alloy based orthopedicimplant that is not prone to corrosion by the action of the body fluidsand is biocompatible and stable over the lifetime of the recipient.

The invention provides a method for forming a uniformly thick oxidecoating on zirconium or a zirconium alloy, each having a refinedmicrostructure, by inducing an altered surface roughness on thezirconium or zirconium alloy, prior to oxidizing the zirconium orzirconium alloy to form a blue-black zirconium oxide coating of uniformand controlled thickness. The invention also provides a method forforming a uniformly thick oxide coating on a zirconium or zirconiumalloy prosthesis, for implantation in a patient, by inducing an alteredsurface roughness on at least a portion of the zirconium or zirconiumalloy prosthesis, wherein the zirconium or zirconium oxide has a refinedmicrostructure, prior to oxidizing the prosthesis to form a blue-blackzirconium oxide coating of uniform and controlled thickness on at leasta portion of the surface of the prosthesis.

FIG. 1 is a schematic diagram depicting a hip joint prosthesis inposition.

FIG. 2 is a schematic diagram showing a typical hip joint prosthesis.

FIG. 3 is a schematic diagram of a knee joint prosthesis in place.

FIG. 4 is a schematic diagram of the parts of a typical knee joint.

One aspect of the present invention is to provide a method for formingan oxide coating of uniform thickness on zirconium or a zirconium alloy,the zirconium or zirconium alloy each having a refined microstructureand an altered surface roughness. Another aspect of the presentinvention is to provide a low friction, wear resistant oxide coating ofuniform thickness on prosthesis surfaces, such as articulating surfacesand irregular surface structures adapted to accommodate tissue ingrowthon a portion of the prosthesis body.

The here-claimed method of forming an oxide coating of uniform thicknessby inducing an altered surface roughness on zirconium or a zirconiumalloy, each having a refined microstructure, prior to oxidizing thezirconium or zirconium alloy is applicable to various prosthetic partsand devices. These prosthetic parts and devices include, but are notlimited to, cardiovascular implants including heart valves, totalartificial heart implants, ventricular assist devices, vascular graftsand stents; electrical signal carrying devices such as pacemaker andneurological leads, and defibrillator leads; guide wires and catheters;percutaneous devices; and joint prostheses including hip joints orsurface replacements, knee joints, shoulder joints, elbows,endoprostheses, spinal segments, and fingers. Illustrative examples ofsuch articulating surfaces are shown in the schematic diagrams, FIGS.1-4.

A typical hip joint assembly is shown m situ in FIG. 1. The hip jointstem 2 fits into the femur while the femoral head 6 of the prosthesisfits into and articulates against the inner lining 8 of an acetabularcup 10 which in turn is affixed to the pelvis as shown in FIG. 1. Aporous metal bead or wire mesh coating 12 may be incorporated to allowstabilization of the implant by ingrowth of surrounding tissue into theporous coating. Similarly, such a porous metal bead or wire mesh coatingcan also be applied to the acetabular component. The femoral head 6 maybe an integral part of the hip joint stem 2 or may be a separatecomponent mounted upon a conical taper at the end of the neck 4 of thehip joint prosthesis. This allows the fabrication of a prosthesis havinga metallic stem and neck but a femoral head of some other material, suchas ceramic. This method of construction is often desirable becauseceramics have been found to generate less frictional torque and wearwhen articulating against the UHMWPE lining of an acetabilar cup.Additionally, zirconia ceramic has been shown to produce less wear ofthe UHMPE than alumina. Regardless of the materials, however, thefemoral head articulates against the inner surface of the acetabular cupthereby causing wear and, in the long term, this may necessitateprosthesis replacement. This is especially the case where the femoralhead is of metal and the acetabular cup is lined with an organic polymeror composite thereof. While these polymeric surfaces provide good,relatively low friction surfaces and are biocompatible, they are subjectto wear and accelerated creep due to the frictional heat and torque towhich they are subjected during ordinary use.

A typical knee joint prosthesis is shown in situ in FIG. 3. The kneejoint includes a femoral component 20 and a tibial component 30. Thefemoral component includes condyles 22 which provide the articulatingsurface of the femoral component and pegs 24 for affixing the femoralcomponent to the femur. The tibial component 30 includes a tibial base32 with a peg 34 for mounting the tibial base onto the tibia. A tibialplatform 36 is mounted atop the tibial base 32 and is supplied withgrooves 38 similar to the shape of the condyles 22. The bottom surfacesof the condyles 26 contact the tibial platform's grooves 38 so that thecondyles articulate within these grooves against the tibial platform.While condyles are typically fabricated of metals, the tibial platformmay be made from an organic polymer or a polymer-based composite. Thus,the hard metallic condyle surfaces 26 would articulate against arelatively softer organic composition. This may result in wear of theorganic material, i.e. the tibial platform, necessitating thereplacement of the prosthesis. As in the case of the hip joint, porousbead or wire mesh coatings can also be applied to either the tibial orfemoral components of the knee or both.

The invention provides uniformly thick zirconium oxide coated orthopedicimplants or prostheses fabricated of zirconium or zirconium containingmetal alloys or a thin coating of zirconium or zirconium alloy onconventional orthopedic implant materials. In order to form continuousand useful zirconium oxide coatings of uniform thickness over thedesired surface of the metal alloy prosthesis substrate, the metal alloyshould contain from about 80 to about 100 wt % zirconium, preferablyfrom about 95 to about 100 wt %. Oxygen, niobium, and titanium includecommon alloying elements in the alloy which include often the presenceof hafnium. Yttrium may also be alloyed with the zirconium to enhancethe formation of a tougher, yttria-stabilized zirconium oxide coatingduring the oxidation of the alloy. While such zirconium containingalloys may be custom formulated by conventional methods known in the artof metallurgy, a number of suitable alloys are commercially available.These commercial alloys include among others ZIRCADYNE 705, ZIRCADYNE702 and Zircalloy.

The base zirconium containing metal alloys are fabricated byconventional methods to the shape and size desired to obtain aprosthesis substrate. The shaped zirconium or zirconium alloy must havea refined microstructure such as might be produced by hot forgeconversion of ingot to wrought barstock Zirconium or a zirconium alloywith a grain size of less than ASTM micro-grain size number 10 wouldexemplify an acceptable degree of refined microstructure. One method ofdetermining if a refined microstructure is present in the zirconium orzirconium alloy is to examine the material in transverse section toexamine the secondary phase (β) which should have grains of not largerthan about 2 microns wide and with not more than about a 3 micronseparation; preferably 1 micron wide and a 2 micron separation.Production of such a fine dispersion of multiple phase grains is notlimited to hot forging of barstock and can be accomplished -by otherprocesses including, but not limited to, closed die forging, rapidsolidification and powder consolidation.

The substrate zirconium or zirconium alloy is then subjected to anabrasive surface preparation process that includes, but is not limitedto, grinding, buffing, mass finishing and vibratory finishing. Theabrasive surface preparation process is used to induce an alteredsurface roughness (Ra) of from about 3 microinches to about 25microinches. The appropriate altered surface roughness is induced byaltering the pre-existing surface roughness to an altered surfaceroughness of such a magnitude as alloy, each having a refinedmicrostructure and an appropriately altered surface roughness, issubjected to an oxidation process.

The substrate is then subjected to process conditions which cause thenatural (in situ) formation of a tightly adhered, diffusion-bondedcoating of uniformly thick zirconium oxide on its surface. The processconditions include, for instance, air, steam, or water oxidation oroxidation in a salt bath. These processes ideally provide a thin, hard,dense, blue-black or black, low-friction wear-resistant uniformly thickzirconium oxide film or coating of thicknesses typically on the order ofseveral microns on the surface of the prosthesis substrate. Below thiscoating, diffused oxygen from the oxidation process increases thehardness and strength of the underlying substrate metal.

The air, steam and water oxidation processes are described innow-expired U.S. Pat. No. 2,987,352 to Watson, the teachings of whichare incorporated by reference as though fully set forth. The oxidationprocess applied to zirconium or a zirconium alloy, each having a refinedmicrostructure and an appropriate degree of altered surface roughness,provides a firmly adherent black or blue-black layer of uniformly thickzirconium oxide of highly oriented monoclinic crystalline form. If theoxidation is continued to excess, the coating will whiten and separatefrom the metal substrate. For convenience, the metal prosthesissubstrate may be placed in a furnace having an oxygencontainingatmosphere (such as air) and typically heated at 900°-1300° F. for up toabout 6 hours. However, other combinations of temperature and time arepossible. When higher temperatures are employed, the oxidation timeshould be reduced to avoid the formation of the white oxide.

One of the salt-bath methods that can be used to apply the zirconiumoxide coatings to the metal alloy prosthesis, is the method of U.S. Pat.No. 4,671,824 to Haygarth, the teachings of which are incorporated byreference as though fully set forth. The salt-bath method provides asimilar, slightly more abrasion resistant blue-black or black zirconiumoxide coating. This method requires the presence of an oxidationcompound capable of oxidizing zirconium in a molten salt bath. Themolten salts include chlorides, nitrates, cyanides, and the like. Theoxidation compound, sodium carbonate, is present in small quantities, upto about 5 wt %. The addition of sodium carbonate lowers the meltingpoint of the salt. As in air oxidation, the rate of oxidation isproportional to the temperature of the molten salt bath and the '824patent prefers the range of 550°-800° C. (1022°-1470° F.). However, thelower oxygen levels in the bath produce thinner coatings than forfurnace air oxidation at the same time and temperature. A salt bathtreatment at 1290° F. for four hours produces an oxide coating thicknessof roughly 7 microns.

Creation of a uniform oxide coating during the oxidation process, by thehere claimed method, is dependent on both a surface with appropriatealtered surface roughness and a microstructure with sufficientrefinement. The oxide coating initiates and grows from surfaceasperities, so the oxide initiation sites may be spaced too far apart toproduce a uniform coating thickness on a surface that is too smooth. Theoxide layer grows by oxygen diffusion along grain boundaries and throughmicrostructural grains. The oxidation rate can be different in grains ofdifferent structure and composition (such as between alpha and betagrains in a two-phase zirconium alloy). Thus, the oxide coating may notgrow with a uniform thickness through a microstructure that is toocoarse. Specific limits for the necessary minimum surface roughness andmaximum microstructural refinement can be alloy and applicationdependent.

The uniformly thick zirconium oxide coating may range up to about 10microns. It is preferred that a uniformly thick blue-black zirconiumoxide layer ranging in thickness from about 1 to about 8 microns shouldbe formed. it is most preferred that the uniformly thick zirconium oxidelayer range from about 3 microns to about 7 microns. For example,furnace air oxidation at 1100° F. for 3 hours will form a uniform oxidecoating of a thickness of 4-5 microns on ZIRCADYNE 705 with a surfaceroughness (Ra) of about 4 microinches. Longer oxidation times and higheroxidation temperatures will increase this thickness, but may compromisecoating integrity. For example, one hour at 1300° F. will form an oxidecoating about 14 microns in thickness, while 21 hours at 1000° F. willform an oxide coating thickness of about 9 microns. Of course, becauseonly a thin oxide is necessary on the surface, only very smalldimensional changes, typically less than 10 microns over the thicknessof the prosthesis, will result. In general, thinner coatings (1-8microns) have better attachment strength.

Blue-black zirconium oxide coatings produced by any of the prior artmethods are quite similar in hardness. For example, if the surface of awrought ZIRCADYNE 705 (Zr, 2-3 wt % Nb) prosthesis substrate isoxidized, the hardness of the surface shows a dramatic increase over the200 Knoop hardness of the original metal surface. The surface hardnessof the blue-black zirconium oxide surface following oxidation by eitherthe salt bath or air oxidation process is approximately 1200-1700 Knoophardness.

These diffusion-bonded, low friction, highly wear resistant, uniformlythick zirconium oxide coatings can be applied to the surfaces oforthopedic implants subject to conditions of wear and to prostheticimplants and devices requiring a biocompatible surface. Such surfacesinclude the articulating surfaces of knee joints, elbows and hip joints.As mentioned before, in the case of hip joints, the femoral head andstem are typically fabricated of metal alloys while the acetabular cupmay be fabricated from ceramics, metals or organic polymer-lined metalsor ceramics.

When the zirconium oxide coatings are applied to surfaces subject towear, it is desirable to obtain a smooth finished surface to minimizeabrasive wear. After the oxidation process, the oxide coating surfacecan be polished by any of a variety of conventional finishingtechniques. Sufficient oxide thickness must be produced to accommodatethe chosen finishing technique. For example, a surface with a uniformoxide coating of about 5 microns thick that had a pre-oxidation surfaceroughness (Ra) of about 4 microinches can be burnished to a finalsurface roughness (Ra) of about 2 microinches with a loss of about 1micron in oxide thickness.

Zirconium or zirconium alloy can also be used to provide a porous beador wire mesh surface to which surrounding bone or other tissue mayintegrate to stabilize the prosthesis. These porous coatings can betreated simultaneously by the oxidation of the base prosthesis for theelimination or reduction of metal ion release. Furthermore, zirconium orzirconium alloy can also be used as a surface layer applied overconventional implant materials prior to inducing an altered surfaceroughness, in situ oxidation and formation of the uniform zirconiumoxide coating.

The process of the present invention avoids the problems of formation ofthick oxide coatings of low abrasion resistance and of significantdimensional changes of the process in U.S. Pat. No. 3,615,885. Thepresent invention also produces an oxide film that is highly abrasionresistant, unlike that of the '885 patent.

The process of the present invention, by inducing an altered surfaceroughness on zirconium or a zirconium alloy, each having a refinedmicrostructure, results in the formation of a blue-black zirconiumdioxide coating of uniform thickness, the depth of which can becontrolled by the proper choice of the oxidation conditions. Theformation of a uniformly thick oxide coating provides an oxide coatingof variable and controlled thickness with especially high abrasion.resistance and reduced wear due to high integrity of the adhesionbetween the oxide layer and the underlying zirconium or zirconium alloyand the high integrity of the adhesion within the oxide layer. The term“high integrity” denotes an oxide coating that is uniform in thicknesswith no visible cracks or pores when viewed in cross-section by opticalmicroscopy.

The invention provides a zirconium or zirconium-containing metal alloyprosthesis with a refined microstructure coated via in situ oxidationwith a zirconium oxide of uniform thickness. The uniformly thickzirconium oxide coating provides the invention prosthesis with a thin,dense, low friction, high integrity, wear resistant, biocompatiblesurface ideally suited for use on articulating surfaces of jointprostheses wherein a surface or surfaces of the joint articulates,translates or rotates against mating joint surfaces. The uniformly thickzirconium oxide coating ,nay therefore be usefully employed on thefemoral heads or inside surfaces of acetabular cups of hip-jointimplants or on the articulating surfaces of other types of prostheses,such as knee joints.

When a joint surface coated with a uniformly thick zirconium oxide isemployed in a manner wherein it articulates or rotates against anon-metallic or non-zirconium oxide coated surface, the low frictioncharacteristic and high integrity of the uniformly thick coating causesreduced friction, wear, and heat generation relative to prior artprostheses. This reduced heat generation results in a lowered tendencyfor the non-metallic or non-zirconium oxide coating bearing surface toexperience creep and torque so that the useful life of the opposingsurface is enhanced. Organic polymers, such as UHMWPE, exhibit rapidlyincreased rates of creep when subjected to heat with consequentdeleterious effect on the life span of the liner. Wear debris of thepolymer leads to adverse tissue response and loosening of the device.Thus, not only does the uniformly thick zirconium oxide coating serve toimprove the protection of the prosthesis substrate to which it isapplied due to its high integrity, it also, as a result of its lowfriction surface, protects those surfaces against which it is inoperable contact and consequently enhances the performance and life ofthe prosthesis.

A uniformly thick zirconium oxide coated joint surface also enhances theuseful life of the opposing surface when the opposing surface is bodytissue. The surgical replacement of one component of the joint is termed“hemiarthroplasty” and because the repaired joint has only oneartificial (prosthesis) component, the artificial component is oftentermed a “unipolar” prosthesis, or “endoprosthesis.” The uniformly thickzirconium oxide coating is a low friction surface for articulation,translation and rotation against body tissue thereby having the samebeneficial effect for a body tissue counterface as it does for anorganic polymer counterface.

The usefulness of zirconium oxide coated prosthesis is not limited toload bearing prostheses, especially joints, where a high rate of wearmay be encountered. Because the uniformly thick zirconium oxide coatingis especially firmly bonded to the zirconium alloy prosthesis substrate,it provides an enhanced barrier between the body fluids and thezirconium alloy metal thereby preventing the corrosion of the alloy bythe process of ionization and its associated metal ion release comparedto non-uniform oxide coatings.

Additionally, the natural in situ formation of a uniformly thickzirconium oxide coating from the presence of zirconium in the substratemetal involves oxygen diffusion into the metal substrate below the oxidecoating. Oxygen, an alloying constituent in zirconium, increases thestrength of the metal substrate, particularly the fatigue strength.Furthermore, the high integrity of the uniformly thick coating reducesthe number of fatigue crack initiation sites relative to a non-uniformlythick oxide coating that contains cracks or pores. Resistance to fatigueloading is paramount in many orthopedic implant applications such as thehip stem, and femoral and tibial knee components. Thus, not only doesthe formation of the uniformly thick zirconium oxide coating improvewear, friction, and corrosion resistance, it also improves themechanical integrity of the implant device from a strength standpoint.

Although the invention has been described with reference to itspreferred embodiments, those of ordinary skill in the art may, uponreading this disclosure, appreciate changes and modifications which maybe made and which do not depart from the scope and spirit of theinvention as described above or claimed hereafter.

What is claimed is:
 1. A method of producing an oxide coating onzirconium or zirconium alloy, the zirconium or zirconium alloy having arefined microstructure of a grain size of less than ASTM micro-grainsize number 10 and a surface roughness, the method comprising: alteringsaid surface roughness of said zirconium or zirconium alloy such that anoxidation process applied to said zirconium or zirconium alloy resultsin the formation of a uniformly thick oxide coating.
 2. A method ofproducing an oxide coating of uniform thickness on zirconium orzirconium alloy, the zirconium or zirconium alloy each having a refinedmicrostructure and a surface roughness, the method comprising: alteringsaid surface roughness such that an altered surface roughness (Ra) is inthe range of about 3 microinches to about 25 microinches prior tosubjecting said zirconium or zirconium alloy to an oxidation process. 3.The method of claims 1 or 2, wherein an altered surface roughness isinduced on the zirconium or zirconium alloy surface by an abrasivesurface preparation process comprising a grinding step.
 4. The method ofclaims 1 or 2, wherein said altered surface roughness (Ra) is in therange of from about 3.5 microinches to about 7 microinches.
 5. Themethod of claims 1 or 2, wherein said oxidation process utilizes air asan oxidant.
 6. The method of claims 1 or 2, wherein the zirconium orzirconium alloy is produced by a process selected from a groupconsisting of hot forge conversion of ingot to barstock, closed dieforging, rapid solidification and powder consolidation.
 7. The method ofclaims 1 or 2, wherein said altered surface roughness (Ra) is in therange of from about 3.5 microinches to about 7 microinches and thezirconium alloy has a grain size of less than ASTM micro-grain sizenumber
 10. 8. The method of claims 1 or 2, wherein altered said surfaceroughness (Ra) is in the range of from about 3.5 microinches to about 7microinches and the zirconion or zirconium alloy is produced by aprocess selected from the group consisting of hot forge conversion ofingot to barstock, closed die forging, rapid solidification and powderconsolidation.
 9. The method of claims 1 or 2, wherein the uniform oxidecoating has a thickness up to about 10 microns.
 10. A prosthesis forimplantation in a patient, comprising: (a) a prosthesis body formed ofzirconium or zirconium alloy comprising an implant portion for insertinginto the body tissue of the patient; (b) a bearing surface comprising atleast one condyle on the prosthesis body; (c) a tibial component formedof an organic polymer or polymer-based composite and adapted tocooperate with the bearing surface; and (d) a thin coating of blue-blackor black zirconium oxide of uniform thickness prepared by the method ofclaims 1 or 2 directly on the bearing surface of the condyle portion forreducing wear of the organic polymer or polymer-based compositecomponent.
 11. The prosthesis of claim 10, wherein said thin blue-blackor black zirconium oxide coating is up to about 10 microns thick. 12.The prosthesis of claim 11, wherein the implant portion of theprosthesis body further comprises an irregular surface structure adaptedto accommodate tissue ingrowth on a portion of the prosthesis body. 13.The prosthesis of claim 12, wherein the irregular surface structure isformed of zirconium or zirconium alloy beads attached to the outersurface of the prosthesis body, wherein at least a portion of thesurface of the beads is oxidized to blue-black or black zirconium oxideof uniform thickness by the method of claims 1 or
 2. 14. The prosthesisof claim 12, wherein the irregular surface structure is formed ofzirconium or zirconium alloy wire mesh connected to the outer surface ofthe prosthesis body, wherein at least a portion of the surface of themesh is oxidized to blue-black or black zirconium oxide of uniformthickness by the method of claims 1 or
 2. 15. A prosthesis forimplantation in a patient, comprising: (a) a hip prosthesis body forimplantation into a femor comprising a head potion formed of zirconiumor zirconium alloy; (b) a bearing surface on the head portion of theprosthesis body; (c) an acetabular cup having an inner surface formed ofan organic polymer or a polymer-based composite, said inner surfacebeing adapted to cooperate with the bearing surface on the head portion;and (d) a thin coating of blue-black or black zirconium oxide of uniformthickness prepared by the method of claims 1 or 2 directly on thebearing surface of the head portion for reducing wear of the acetabularcup inner surface.
 16. The prosthesis of claim 15, wherein said thinblue-black or black zirconium oxide coating of uniform thickness is upto about 10 microns thick.
 17. The prosthesis of claim 15, wherein theprosthesis body further comprises an irregular surface structure adaptedto accommodate tissue ingrowth on a portion of the prosthesis body. 18.The prosthesis of claim 17, wherein the irregular surface structure isformed of zirconium or zirconium alloy beads connected to the outersurface of the prosthesis body, wherein at least a portion of thesurface of the beads is oxidized to blue-black or black zirconium oxideof uniform thickness by the method of claims 1 or
 2. 19. The prosthesisof claim 17, wherein the irregular surface structure is formed ofzirconium or zirconium alloy wire mesh connected to the outer surface ofthe prosthesis body, wherein at least a portion of the surface of themesh is oxidized to blue-black or black zirconium oxide of uniformthickness by the method of claims 1 or
 2. 20. A prosthesis forimplantation in a patient, comprising: (a) a prosthesis body formed ofzirconium or zirconium alloy comprising an implant portion for insertioninto the body tissue of the patient; (b) a bearing surface on theprosthesis body, the bearing surface being sized and shaped to engage orcooperate with a second bearing surface on another prosthesis portion,said second bearing surface being formed of an organic polymer orpolymer-based composite; and (c) a coating, formed by the method ofclaims 1 or 2, of blue-black or black zirconium oxide of uniformthickness up to about 10 microns in thickness on the bearing surface ofthe prosthesis body for reducing wear on the organic polymer orpolymer-based second bearing surface of said another prosthesis portion.21. The prosthesis of claim 20, wherein the prosthesis body is a hipjoint having a head portion as a bearing surface and wherein saidanother prosthesis portion is an acetabular cup, said head portion beingadapted to cooperate with the inner surface of the acetabular cup, saidinner surface comprising an organic polymer or polymer-based composite.22. The prosthesis of claim 20, wherein the prosthesis body is a kneejoint and the bearing surface of the prosthesis body comprises at leastone condyle, and wherein said another prosthesis portion comprises atibial component formed of an organic polymer or polymer-basedcomposite, said at least one condyle being adapted to cooperate with thetibial component.
 23. The prosthesis of claim 20, wherein the prosthesisbody further comprises an irregular surface structure adapted toaccommodate tissue ingrowth on a portion of the prosthesis body.
 24. Theprosthesis of claim 23, wherein the irregular surface structure isformed of zirconium or zirconium alloy beads connected to the outersurface of the prosthesis body, wherein at least a portion of thesurface of the beads is oxidized to blue-black or black zirconium oxideof uniform thickness by the method of claims 1 or
 2. 25. The prosthesisof claim 23, wherein the irregular surface structure is formed ofzirconium or zirconium alloy wire mesh connected to the outer surface ofthe prosthesis body, wherein at least a portion of the surface of themesh is oxidized to blue-black or black zirconium oxide of uniformthickness by the method of claims 1 or
 2. 26. A prosthesis forimplantation in a patient, comprising: (a) a prosthesis body formed ofzirconium or zirconium alloy comprising an implant portion for insertinginto the body tissue of the patient; (b) a bearing surface on theprosthesis body; (c) a counter-bearing surface formed of an organicpolymer or polymer-based composite and adapted to cooperate with thebearing surface; and (d) a thin coating of blue-black or black zirconiumoxide of uniform thickness prepared by the method of claims 1 or 2directly on the bearing surface for reducing wear of the organic polymeror polymer-based composite counter-bearing surface.
 27. The prosthesisof claim 26, wherein said thin blue-black or black zirconium oxidecoating of uniform thickness is up to about 10 microns thick by themethod of claims 1 or
 2. 28. The prosthesis of claim 27, wherein theimplant portion of the prosthesis body further comprises an irregularsurface structure adapted to accommodate tissue ingrowth on a portion ofthe prosthesis body.
 29. The prosthesis of claim 28, wherein theirregular surface structure is formed of zirconium or zirconium alloybeads attached to the outer surface of the prosthesis body, wherein atleast a portion of the surface of the beads is oxidized to blue-black orblack zirconium oxide of uniform thickness by the method of claims 1 or2.
 30. The prosthesis of claim 28, wherein the irregular surfacestructure is formed of zirconium or zirconium ally wire mesh connectedto the outer surface of the prosthesis body, wherein at least a portionof the surface of the mesh is oxidized to blue-black or black zirconiumoxide of uniform thickness by the method of claims 1 or
 2. 31. Theprosthesis body of claims 10, 15, 20 or 26 wherein the zirconium orzirconium alloy is wrought barstock.
 32. A prosthesis for implementationin a patient, comprising: (a) a prosthesis body having an externalsurface at least a portion of which is formed of zirconium or zirconiumalloy, having a refined microstructure of a grain size of less than ASTMmicro-grain size number 10 and an altered surface roughness (Ra) in therange of about 3 microinches to about 25 microinches; and (b) azirconium oxide coating of uniform thickness formed on said portion ofthe external surface by inducing an altered surface roughness on atleast said portion of the external surface and subjecting said portionof the external surface of said prosthesis body to an oxidation process.33. The prosthesis of claim 32 wherein the prosthesis body is anendoprothesis body suitable for use in a joint selected from a groupconsisting of knee joint, hip joint and shoulder joint.
 34. Theprosthesis of claims 33, wherein the zirconium or zirconium alloy isformed by a process selected from a group consisting of hot forgeconversion of ingot to barstock, closed die forging, rapidsolidification and powder consolidation.